This project is based on our discovery that stimulation of the antigen receptor of mature T cells following exposure of the cells to mitogenic lymphokines such as IL-2 leads to the induction of programmed cell death or apoptosis (propriocidal regulation). We are interested in the mechanism of propriocidal regulation. We are also studying whether this mechanism can explain certain phenomenon of immunological "suppression" that have been previously studied and how this mechanism plays a role in various immunomodulatory strategies being attempted for human disease. We have begun to explore the manner in which specific ways in which antigen engages the receptor affect whether or not apoptosis is induced. Using a panel of peptide variants in which different amino acids are substituted at position 99 in the 88 to 104 peptide of pigeon cytochrome c, we have found variants that will induce no IL-2 or other cytokines but still initiate T cell apoptosis. Further investigation of two such "altered peptide ligands" has shown that these do not induce the same tyrosine phosphorylation patterns of the CD3 chain, the TCRzeta chains, and the ZAP70 kinase as the 88-104 peptide. Instead, the death-inducing altered peptide ligands cause only one form of TCRzeta phosphorylation and a novel type of phosphorylation of the fyn kinase molecule. These results indicate that qualitatively different types of T cell receptor engagement may induce apoptosis through distinct signalling pathways. These findings also hold promise that certain altered peptide ligands may be potent tolerogens without a risk of causing T cell activation, which may lead to improved apoptosis-inducing therapies for T cell-mediated diseases. We have also carefully studied the types of cysteine proteases that are induced during propriocidal death. We have found that CPP32beta and ICELAP3 but not interleukin-1beta converting enzyme (ICE) are proteolytically activated during T cell death. This differs from other cell-types in which ICE appears to be activated first and play a role in the activation of CPP32beta. We also found that specific inhibitors of CPP32beta, but not ICE will block T cell apoptosis. Similar studies are now being carried out in HIV-infected T cells to determine whether these proteases take part in the cytolytic effects of HIV. These investigations will advance our understanding of the molecular mechanism of the propriocidal regulatory pathway.